1. Field of the Invention
The present invention relates to a flat-plate antenna used in mobile communications and more particularly to a flat-plate antenna mounted to a vehicle body and used for automobile telephones of MCA (multi-channel access), etc.
2. Prior Art
Various types of wire-form antennas have conventionally been used as automobile communication antennas. The reason for this is that wire-form antennas have maximum radiating characteristics in the horizontal direction (which is required for mobile communications) and can easily be endowed with characteristics which are non-directional in a horizontal plane.
Furthermore, antennas used for automobile telephones and MCA require broad-band characteristics, and for the wire-form antennas, techniques for obtaining such broad-band characteristics have been established so that such demands can be met relatively easily in antenna development and design.
In recent years, flat-plate antennas have attracted attention as antennas for mobile communications. The reason for this is that such antennas provide very considerable operating merits.
More specifically, when a flat-plate antenna is attached to an automobile, there is no projected object on a vehicle; accordingly, there is no deleterious effect on the style of the vehicle. Furthermore, wind noise is remarkably reduced during the operation of the vehicle, and antenna damage is less likely to occur since there is no danger for the flat-plate antenna to contact with car wash machinery, garages, roadside trees, etc.
When flat-plate antennas are used, the antennas must have broad-band characteristics. For this reason, antennas having multi-layer structures have been proposed. However, since such proposed antennas are too complex in structure to be formed in an integral unit, it has been difficult to commercialize the antennas.